The following abbreviations are herewith defined:
3GPP third generation partnership project
CSI channel state information
DM diversity multiplexing
DVB-H digital video broadcast for handheld devices
eNB evolved node B (of an LTE system)
E-UTRAN evolved UTRAN (also known as LTE or 3.9 G)
i.i.d independent and identically distributed
LTE long term evolution of 3GPP
MI mutual information
Node B base station or similar network access node
UE user equipment (e.g., mobile equipment/station)
UMTS universal mobile telecommunications system
UWB ultra wideband
UTRAN UMTS terrestrial radio access network
WiMAX world interoperability for microwave access
Wireless relay networks are the environment of these teachings, such as for example WiMAX, E-UTRAN (also known as 3.9 G), UWB systems and DVB-H. Certain wireless relaying offers benefits such as easy and fast network deployment, low cost of installation and maintenance, flexibility, and scalability in both size and density. Coverage probability increases exponentially with the number of relay nodes in the network. Additionally, the use of multiple relays for a single message increases diversity in the signal.
The following papers employ system models not unlike the relay environment of these teachings. J. Nicholas Laneman and Gregory W. Wornell [DISTRIBUTED SPACE-TIME-CODED PROTOCOLS FOR EXPLOITING COOPERATIVE DIVERSITY IN WIRELESS NETWORKS; IEEE Transactions on Information Theory, vol. 49, no. 10, pp. 2415-2425, October 2003] proposes space-time coded cooperative diversity protocols achieving full spatial diversity gain (i.e., the diversity order equals the number of relay terminals). Y. Jing and B. Hassibi [DISTRIBUTED SPACE-TIME CODING IN WIRELESS RELAY NETWORKS; IEEE Transactions on Wireless Communication, vol. 5, no. 12, pp 3524-2536, December 2006] analyzes distributed linear dispersion space-time coding schemes and show that a diversity order equal to the number of relay terminals can be achieved. Kambiz Azarian, Hesham el Gamal and Philip Schniter [ON THE ACHIEVABLE DIVERSITY-MULTIPLEXING TRADEOFF IN HALF-DUPLEX COOPERATIVE CHANNELS; IEEE Transactions on Information Theory, vol. 51, no. 12, pp. 4152-4172, December 2005] assume the presence of a direct link between source and destination, and shows that an extension to the multi-relay case previously introduced in R. U. Nabar, H. Bolcskei, and F. W. Kneubuhler [FADING RELAY CHANNELS: PERFORMANCE LIMITS AND SPACE-TIME SIGNAL DESIGN; IEEE J. Selected Areas of Communication, vol. 22 no. 6, pp 1099-1109, August 2004] is optimal for a diversity-multiplexing (DM) tradeoff.
But in addition to the above specific teachings as to enhancing diversity, there is the practical problem of deciding which of several relay nodes are best suited for acting as relay between a particular source and destination, in particular in the presence of several source nodes and when the relay resources need to be shared by the source nodes. As relay nodes become more prevalent there will be occasions where two or more relay nodes have the capability (due to physical positioning, quality of signal received from the source, available bandwidth/radio resources, etc.) to act as relay for a particular communication. Allowing or requiring each node that can potentially relay a received communication from a source to its destination to actually relay that communication is seen to lead in some instances to diminished data throughput in the cell. While employing all available/capable relays for a single source-to-destination communication may increase diversity for that communication, it is clear that certain diversity increases would surpass the point of diminishing returns and not provide a sufficiently offsetting benefit. Furthermore, as stated above, often the relay nodes cannot be used simultaneously by all concurrent sources, and thus their use has be controlled in a fair an efficient manner to increase the lifetime of the network and to improve the capacity of the relay network.
Relevant teachings in this respect may be seen at a paper by A. Hottinen and T. Heikkinen [DELAY-DIFFERENTIATED SCHEDULING IN A RANDOMIZED MIMO RELAY NETWORK; EUSIPCO, Poznań 2007]. This paper details the destination as scheduling the transmission of possibly delay-differentiated services of M sources which go through R relays in a manner that accounts for both efficiency and fairness in the scheduling. In that paper the system model assumes relays having only one transmit and receive antenna while the overall relay network (source-to-destination channel) is MIMO (the sources and destination nodes employ multiple antennas in their communications). In this paper all relays are used by the source that is scheduled.
In practice, all potential relay nodes cannot serve a single source node. In such a case, relay selection problem occurs. The system has to decide how to optimally share the available relays among the competing source nodes. Relay selection that is optimal in terms of network throughput is a difficult problem and no optimal solutions are seen in the prior art. Iterative greedy solutions exist where the best relays are selected in a round-robin fashion by transmitters or the network. What is needed in the art is a way to select one or more relay nodes from among a plurality of available relay nodes for relaying a communication between source and destination in a manner that accounts for data throughput in the cell.